1. Field of the Invention
The present invention relates generally to an optical deflector device utilizing an electro-optical effect, and more particularly to a technique for minimizing a drift of a deflecting angle of a light beam, relative to a voltage applied to the device for controlling the deflecting angle.
2. Discussion of the Prior Art
There is known an optical deflector device which includes a substrate having an electro-optical effect and capable of guiding light rays therethrough, a buffer layer formed on one major surface of the substrate, and a plurality of deflection electrodes formed on the buffer layer to produce an electric field within the substrate. In this type of optical deflector, the light rays passing through the substrate are deflected in response to a change in the distribution of the refractive index of the substrate according to a controlled deflection voltage to be applied between the electrodes.
In such a known type of optical deflector device, the buffer layer provided on one of the opposite major surfaces of the substrate is usually formed as a single film of a suitable material such as silicon oxide (SiO.sub.2), by a sputtering or other suitable method. The electrodes are then formed on the thus formed buffer layer, so that the buffer layer prevents the electrodes from absorbing the energy of the light rays passing through the substrate. An extensive study of this known deflector device revealed that the buffer layer formed as a single continuous film tends to easily lack oxygen and become electrically active at its local portions during operation. In this condition, an electric charge is able to move between the deflection electrodes through the electrically activated portion of the buffer layer, while a deflection voltage is applied between the electrodes. As a result, the electric field produced between the electrodes is affected by the movement of the electric charge, whereby the deflection angle is deviated from a set value determined by the controlled deflection voltage. Thus, the known optical deflector indicated above suffers from a "DC drift", i.e., an error in the deflection angle of the light rays with respect to the controlled reflection voltage.